1. Field of the Invention
The present invention relates to a packet replenishing method used when a packet is transmitted or received over a wireless communications network, and a mobile node, a relay node, and a receiving terminal used in the wireless communications network.
2. Description of the Related Art
FIG. 1 shows an example of the configuration of an existing IPv6 network. A home agent (mobile support device, HA) 6, access routers 7 and 8, and a router 9 are connected to the Internet 10. A mobile node (MN) 2 transmits data to a corresponding node CN) 5. The mobile node 2 belongs to a wireless network A and communicates with the corresponding node 5 through the access router 7. After the mobile node 2 transfers to belong to a wireless network B, it performs communications through the access router 8.
FIG. 2 is an explanatory view showing an operation performed when handover occurs. First in (1), when the mobile node 2 enters the wireless network A before the transfer, it receives a router advertisement RA1. In (2), the mobile node 2 generates a care of address CoA-7. In (3), the mobile node 2 makes position registration for a home agent HA6. The home agent HA6 manages home address HoA-2 of the mobile node 2. In (4), the home agent HA6 associates the home address HoA-2 of the mobile node 2 with the care of address CoA-7 using the binding cache, thereby managing the information that the mobile node 2 belongs to the wireless network A. In (5), the mobile node 2 transfers from the wireless network A to the wireless network B under the access router 8, thus bringing about handover.
In (6), the mobile node 2 receives a router advertisement RA2 from the access router 8. In (7), the mobile node 2 generates a care of address CoA-8. In (8), the mobile node 2 makes position registration for the home agent HA6. In (9), the home agent HA6 associates the home address HoA-2 of the mobile node 2 with the care of address CoA-8 generated by the mobile node 2 in (7) using the binding cache of the mobile node 2 and manages them.
FIG. 3 is an explanatory view showing the problem of the conventional technology performed when handover occurs. (1) through (4) shown in FIG. 3 are the operations similar to those in (1) through (4) shown in FIG. 2.
(5) shown in FIG. 3 is data to be transmitted by the mobile node 2 to the corresponding node 5 in several transmitting operations. Numbers 1 through 7 in (5) are sequential numbers of packets storing divided transmitted data, and transmitted in the ascending order.
In (6), the mobile node 2 sequentially transmits packets storing transmitted data. In this process, the mobile node 2 is assumed to have transmitted the packets up to the packet 3. (7) indicates the data received by the corresponding node 5. The corresponding node 5 receives the packets 1 and 2, and waits for the packet 3.
By the transfer of the mobile node 2, the handover of (8) occurs. Since the handover has occurred during the transmission of the packet 3, the packet 3 does not reach the corresponding node 5. The operations (9) through (12) performed when the handover occurs are similar to the operations (6) through (9) shown in FIG. 2.
In FIG. 3, after the processes (9) through (12) are performed, the mobile node 2 starts transmitting data to the corresponding node 5 again in (13). The mobile node 2 recognizes that it has transmitted the packets up to the packet 3. Therefore, the packets are transmitted from the packet 4 in (13).
(14) refers to the data received by the corresponding node 5 in the operations up to the operation (13). When it is recognized that the corresponding node 5 has not received the packet 3, flow control is performed when a packet loss has occurred between the mobile node 2 and the corresponding node 5 in (15).
As the first method of preventing the above-mentioned flow control disclosed by the Japanese Patent Application Laid-open No. Hei 10-308775, there is a well-known method of starting accumulating transmitted packets on the terminal side if it is determined that handover is required when a mobile node transfers, and transmitting the accumulated packets after the handover terminates and the communications are resumed. The first method is explained below by referring to FIG. 4.
The operations (1) through (4) shown in FIG. 4 are the same as the operations (1) through (4) shown in FIG. 3.
(5) refers to the data to be transmitted. In (6), the mobile node 2 transmits the packets 1 and 2 through the access router 7 which provides the wireless network A. (7) refers to the corresponding node 5 receiving the packets 1 and 2.
The handover occurs in (8). At this time, the mobile node 2 buffers the packet 3 and the subsequent packets in the packets indicated in (5), and stops the transmission of packets. The operations in (9) through (12) to be performed when the handover has occurred are the same as the operations (6) through (9) shown in FIG. 3.
After the completion of the processes in (9) through (12) after the mobile node 2 transfers to the wireless network B, the packet 3 and the subsequent packets buffered in (8) above are transmitted to the corresponding node 5. Thus, a loss of a packet can be avoided.
As the second method of preventing the flow control from being performed when the handover occurs as disclosed by Japanese Patent Application Laid-open No. 2002-223467, there is a well-known method of stopping the transmission of a packet when the packet numbers transmitted by performing dynamic delay ACK control when the handover occurs are inverted. The process is explained below by referring to FIG. 5.
Most of the procedure shown in FIG. 5 is the same as the procedure shown in FIG. 3. However, in the procedure shown in FIG. 5, it is assumed that there is a delay difference between the wireless network A and the wireless network B. In this example, the delay difference between the wireless network A and the home agent HA6 is assumed to be larger than that between the wireless network B and the home agent HA6. (1) through (7) shown in FIG. 5 are similar to (1) through (7) shown in FIG. 3. When the handover occurs, the mobile node 2 transmits the packets having data 1 through 3 from the wireless network A to the corresponding node 5. On the other hand, the corresponding node 5 receives only data 1 and 2. The handover occurs in (8). In (9) through (12), the operations up to performing position registration are the same in the wireless network B as those shown in FIG. 3.
In (13), the transmission of packets is started from the wireless network B after the transfer. Based on the delay difference between the wireless network A and the wireless network B, the packet transmitted from the wireless network B side is received the corresponding node 5 earlier than the packet transmitted from the wireless network A side. For example, assume that there is the delay difference of two packets. At this time, when the corresponding node 5 detects that packets have not been received in a correct order, it requests the mobile node 2 to retransmit the packets. However, since the retransmission of packets lowers the throughput of the communications, the problem has to be avoided in the following method.
That is, in (14), when handover occurs, the corresponding node 5 delays the transmission of ACK (“Acknowledgement”) when packets are received. In FIG. 5, for example, the transmission of ACK is delayed by three packets. Thus, although the order of the packets received by the delay difference in the wireless network is inverted, a retransmission request is not immediately issued, thereby preventing the execution of the flow control.
In the above-mentioned first method, it is always necessary to expect the occurrence of handover on each mobile node. When the occurrence of handover is expected, accumulating the packets being transmitted is started, and the transmission of packets is stopped. When the process of the handover is completed, the transmission of accumulated packets is resumed. However, it is difficult to predict the occurrence of handover on each mobile node. If it should be possible, the load on the network becomes very heavy.
Although the strength of a wave becomes weak in a process other than the handover, the transmission of packets from a mobile node is stopped on the network side in case of the occurrence of handover. However, when the transmission of a packet is stopped for any reason other than the handover, there is no opportunity of resuming the transmission of packets. Therefore, the packet to be transmitted by a mobile node cannot reach a corresponding node.
In the above-mentioned second method, the degradation of throughput by executing the flow control can be avoided. However, when a packet is lost, it cannot be replenished.